This invention involves improvements to a wireless a discrete multitone spread spectrum communications system.
Wireless communications systems, such as cellular and personal communications systems, operate over limited spectral bandwidths. They must make highly efficient use of the scarce bandwidth resource to provide good service to a large population of users. Code Division Multiple Access (CDMA) protocol has been used by wireless communications systems to efficiently make use of limited bandwidths. The protocol uses a unique code to distinguish each user""s data signal from other users"" data signals. Knowledge of the unique code with which any specific information is transmitted, permits the separation and reconstruction of each user""s message at the receiving end of the communication channel.
The personal wireless access network (PWAN) system described below, uses a form of the CDMA protocol known as discrete multitone spread spectrum (DMT-SS) to provide efficient communications between a base station and a plurality of remote units. (The term xe2x80x9cdiscrete multitone stacked carrier (DMT-SS)xe2x80x9d also refers to this protocol.) In this protocol, the user""s data signal is modulated by a set of weighted discrete frequencies or tones. The weights are spreading codes that distribute the data signal over many discrete tones covering a broad range of frequencies. The weights are complex numbers with the real component acting to modulate the amplitude of a tone while the complex component of the weight acts to modulate the phase of the same tone. Each tone in the weighted tone set bears the same data signal. Plural users at the transmitting station can use the same tone set to transmit their data, but each of the users sharing the tone set has a different set of spreading codes. The weighted tone set for a particular user is transmitted to the receiving station where it is processed with despreading codes related to the user""s spreading codes, to recover the user""s data signal. For each of the spatially separated antenna array elements at the receiver, the received multitone signals are transformed from time domain signals to frequency domain signals. Despreading weights are assigned to each frequency component of the signals received by each antenna array element. The values of the despreading weights are combined with the received signals to obtain an optimized approximation of individual transmitted signals characterized by a particular multitone set and transmitting location. The PWAN system has a total of 2560 discrete tones (carriers) equally spaced in 8 MHZ of available bandwidth in the range of 1850 to 1990 MHZ. The spacing between the tones is 3.125 kHz. The total set of tones are numbered consecutively from 0 to 2559 starting from the lowest frequency tone. The tones are used to carry traffic messages and overhead messages between the base station and the plurality of remote units. The traffic tones are divided into 32 traffic partitions, with each traffic channel requiring at least one traffic partition of 72 tones.
In addition, the PWAN system uses overhead tones to establish synchronization and to pass control information between the base station and the remote units. A Common Link Channel (CLC) is used by the base to transmit control information to the Remote Units. A Common Access Channel (CAC) is used to transmit messages from the Remote Unit to the Base. There is one grouping of tones assigned to each channel. These overhead channels are used in common by all of the remote units when they are exchanging control messages with the base station.
In the PWAN system, Time Division Duplexing (TDD) is used by the base station and the remote unit to transmit data and control information in both directions over the same multi-tone frequency channel. Transmission from the base station to the remote unit is called forward transmission and transmission from the remote unit to the base station is called reverse transmission. The time between recurrent transmissions from either the remote unit or the base station is the TDD period. In every TDD period, there are four consecutive transmission bursts in each direction. Data is transmitted in each burst using multiple tones. The base station and each remote unit must synchronize and conform to the TDD timing structure and both the base station and the remote unit must synchronize to a framing structure. All remote units and base stations must be synchronized so that all remote units transmit at the same time and then all base stations transmit at the same time. When a remote unit initially powers up, it acquires synchronization from the base station so that it can exchange control and traffic messages within the prescribed TDD time format. The remote unit must also acquire frequency and phase synchronization for the DMT-SS signals so that the remote is operating at the same frequency and phase as the base station.
The PWAN system provides for base station beam steering in the horizontal plane. Interference sources that are located in substantially the same horizontal plane as the base station antenna array can have their effects reduced by steering the received sensitivity direction away from that source. A corresponding horizontal shift can be made in the transmitted beam direction to avoid creating interference at the location of the interfering source. In effect, nulls are steered onto the interfering sources. However, a problem arises when there are interfering sources located in the same radial direction from the base station as a remote unit but are separated in the vertical plane. Such sources cannot be adequately minimized by the PWAN system as the beam and null resolution attained in the horizontal plane is insufficient.
This problem is solved, in accordance with the invention, by providing two or more antenna arrays, here called subarrays arranged in the vertical direction to give spatial adaptivity in the vertical plane to the wireless discrete multitone spread spectrum communications system. The PWAN system is based on a combination of Discrete Multitone Spread Spectrum (DMT-SS) and multi-element adaptive antenna array technologies. The inventors have discovered that the spatial and spectral processing of the DMT-SS signals received or transmitted from the base station antenna array is independent of the type of antenna array utilized. This processing maximized the overall signal-to-interference level of a base station""s coverage area. Using vertically-separated subarrays (composed of horizontal antenna elements) enables the base station to position beams on remote units in the vertical plane as well as position nulls on interferers located at the same azimuth angle as the remote but which are separated in elevation angle. This additional vertical plane adaptivity will enhance the PWAN system""s overall performance and improve the signal-to-interference level.
The invention is a highly bandwidth-efficient communications method to enable vertical and horizontal receive beam steering, that includes the following steps. The base station receives a first spread signal at a base station having a multi-subarray antenna array with a first plurality of subarrays arranged in a spaced vertical direction and a second plurality of subarray elements arranged in a spaced horizontal direction. The first spread signal comprises a first data signal spread over a plurality of discrete tones in accordance with a remote spreading code assigned to a remote unit for a first time period. The base station adaptively despreads the received signal by using first despreading codes that are based on the characteristics of the received signals at the first plurality of subarrays of the array. The vertical displacement of the subarrays enables the base station to perform vertical, receive beam steering. In addition, the base station adaptively despreads the signal received by using second despreading codes that are based on the characteristics of the received signals at the second plurality of subarray elements of the array. The horizontal displacement of the subarray elements enables the base station to perform horizontal, receive beam steering.
The invention also enables transmit beam steering in the vertical and horizontal directions. The method includes the following additional steps. The base station spreads a second data signal with first spreading codes derived from the first despreading codes, that distributes the second data signal over a plurality of discrete tones and the first plurality subarrays of the array, forming a first spectrally spread signal that is spatially spread vertically. The vertical displacement of the subarray enables the base station to perform vertical, transmit beam steering. The base station also spreads the second data signal with second spreading codes derived from the second despreading codes, that distributes the second data signal over the plurality of discrete tones and the second plurality subarray elements of the array, forming a second spectrally spread signal that is spatially spread horizontally. The horizontal displacement of the subarray enables the base station to perform horizontal, transmit beam steering. The base station then transmits the first and second spread signals during a second time period.
The invention has advantageous applications in the field of wireless communications, such as cellular communications or personal communications, where bandwidth is scarce compared to the number of the users and their needs. Such applications may be effected in mobile, fixed, or minimally mobile systems.